You do bring up a good point and one that I haven't thought about, namely, what if a reclearances takes you further away from your ETOPS alternates.
I assumed that this would be easier to determine, simply by plotting the new clearance on your chart. Is it not the case that you can check if it falls within the approved area of operation by comparing it to the range circles already drawn during the planning stage?
I guess my main question is, what is the operational procedure for re-plotting ETPs? I was under the impression that most operators place these in their FMS as a reference fix for situational awareness. If you're no longer flying your planned route, what do you enter? Would using the original lat/long with the <abeam> function activated suffice?